English

• 1. INTRODUCTION

  Thermochromic materials have attracted extensive attention over the past decades because of its practical applications in the fields such as temperature sensors, thermometers and smart window^[1–6]. The exploration for novel materials with thermochromic property has brought various kinds of fascinating chromophores, including inorganic, organic, liquid crystal, polymer and coordination complex compounds. Compared to other materials, coordination complexes represent a rapidly growing class, which provides various choices of porous microstructure, large specific surface area, as well as interesting photophysical properties modulated by peripheral ligand and the central coordination metal ions.

  Many transition metal coordination compounds including those of Pt(Ⅱ)^[7–9], Au(Ⅰ)^[10–12], Ag^{[13, 14]}, Cu(Ⅰ)^[15–21], Zn(Ⅱ)^[22–24], Cd(Ⅱ)^[24], Mn(Ⅱ)^[25] and so on have been reported to display temperature-dependent luminescence response. Among them, Ag(Ⅰ) complexes are attracting more and more attention not only because of the various coordination modes of Ag(Ⅰ) ion, but also due to their photophysical and photochemical properties. However, as compared to the other coin-metal coordination complexes, the luminescent thermo-chromism of silver(Ⅰ) compounds is relatively less reported in the literature. Here we report the assembly of a new luminescent thermochromism silver(Ⅰ) complex through a mixed-ligand system consisting of dicyanamide and phosphine ligand (dppm), namely {Ag₂(dppm)₂[N(CN)₂]}₂-[μ-N(CN)₂]₂ (1). This complex was fully characterized by single-crystal X-ray diffraction, IR, high-resolution mass spectroscopy, thermal analysis and fluorescence spectra. Furthermore, the temperature-depending luminescent property of complex 1 was also investigated detailed.

  2. EXPERIMENTAL

  2.1 Materials

  The reaction was carried out under argon using Schlenk techniques. The solvents were dried and distilled prior to use except that those for spectroscopic measurements were of spectroscopic grade. Ag(CF₃SO₃), bis(diphenylphos-phino)methane (dppm) and sodium dicyanamide were purchased from commercial sources and used as received unless stated otherwise.

  2.2 Physical measurements

  Infrared spectrum (IR) was conducted on a Bruker VERTEX 70 FT-IR spectrophotometer with KBr pellets. Elemental analysis (C, H, N) test was performed on a Perkin-Elmer model 240C elemental analyzer. High resolution mass spectrometry was carried out on a Bruker Impact Ⅱ Q-TOF mass spectrometer using acetonitrile as mobile phases. ¹H NMR spectrum with chemical shifts reported relative to tetramethylsilane was tested using a Bruker AVANCE 400 MHz spectrometer. Thermogravimetric stability analysis was recorded on a NETZSCH STA 449C unit at a heating rate of 10 ℃/min in the nitrogen atmosphere. Analysis of the luminescence properties including emission and lifetime of complex 1 was carried out using an Edinburgh Analytical instrument FLS920 equipped with an Edinburgh Xe900 xenon arc lamp as the exciting light source.

  2.3 Synthesis of complex {Ag₂(dppm)₂[N(CN)₂]}₂[μ-N(CN)₂]₂ (1)

  The synthesis process of complex 1 is demonstrated in Scheme 1. The coordination reaction occurring in dppm, NaN(CN)₂ and Ag(CF₃SO₃) at a 1:1:1 ration produces the crystal of 1. Specifically, to a methanol (10 mL) solution of bis(diphenylphosphino)methane (192.2 mg, 0.50 mmol) was added 128.5 mg Ag(CF₃SO₃) (0.50 mmol). After stirring for 5 min, a methanol (5 mL) solution of NaN(CN)₂ (44.5 mg, 0.50 mmol) was carefully poured to the above solution. After stirring for 1 minute, the previous clear solution becomes a slight suspended solution. Then stop stirring and keep the resulting solution under room temperature in the darkness for three days. The product was obtained as colorless crystals (215 mg, yield: 77%). Anal. Calcd. (%) for C₁₀₈H₈₈Ag₄N₁₂P₈: C, 58.09; H, 3.97; N, 7.53. Found (%): C, 57.54; H, 4.08, N, 7.41. HRMS m/z (%): 2233.1472 (100) [M+H]⁺ (calcd. 2233.1450). IR (KBr, cm⁻¹): 2274 (s, N(CN)₂), 2240 (s, N(CN)₂), 2202 (s, N(CN)₂), 2153 (s, N(CN)₂), 2139 (s, N(CN)₂). ¹H NMR (400 MHz, d₆-DMSO, ppm): δ 7.68~7.60 (m, 32H), 7.36 (t, 16H, J = 7.34 Hz), 7.26 (t, 32H, J = 7.55 Hz), 3.92 (s, 8H).

  Scheme 1

  

  Scheme 1.  Synthetic route of complex {Ag₂(dppm)₂[N(CN)₂]}₂[μ-N(CN)₂]₂ (1)

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  2.4 Crystallographic data collection and refinement

  X-ray crystallographic structure analysis of the single crystal of complex 1 was determined on a Bruker D8 Venture diffractometer by the ω scan mode using graphite-monochromated MoKa (λ = 0.71073 Å) radiation. The APEX Ⅲ software package was used for data reduction and empirical absorption correction. The structure was solved by direct methods with the program SHELXS-97. The heavy atoms were located from E-map, and the rest non-hydrogen atoms were found in the residual Fourier maps. All non-hydrogen atoms were refined with anisotropic displacement parameters. The positions of hydrogen atoms were generated geometrically and refined with isotropic thermal parameters. The structure was refined on F² by full-matrix least-squares approach with the SHELXL-2014 program package^[26]. Crystal data for the title complex are as follows: monoclinic system, space group P2₁/n with a = 13.0303(6), b = 22.2293(9), c = 17.6391(8) Ǻ, β = 98.834(2)º, V = 5048.6(4) Ǻ³, M_r = 2233.14, Z = 2, F(000) = 2256, D_c = 1.469 g/cm³, μ(MoKα) = 0.945 mm^-1, S = 1.087, R = 0.0255 and wR = 0.0938 for 10467 observed reflections with I > 2σ(I). The selected bond lengths and bond angles for complex 1 are listed in Table 1.

  Table 1

  

  Table 1.  Selected Atomic Distances (Å) and Bond Angles (°) of the Cluster Complex 1

  DownLoad: CSV

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Ag(1)–N(1)	2.358(2)		Ag(1)–N(6)#1	2.538(2)		Ag(2)–N(6)#1	2.394(2)
  Ag(1)–P(1)	2.4280(6)		Ag(1)–Ag(2)	3.0990(3)		Ag(2)–P(4)	2.4387(6)
  Ag(1)–P(3)	2.4394(6)		Ag(2)–N(4)	2.389(2)		Ag(2)–P(2)	2.4729(6)
  Angle	(°)		Angle	(°)		Angle	(°)
  N(1)–Ag(1)–P(1)	112.25(6)		P(1)–Ag(1)–Ag(2)	89.642(15)		N(6)#1–Ag(2)–P(2)	105.27(6)
  N(1)–Ag(1)–P(3)	99.73(6)		P(3)–Ag(1)–g(2)	86.128(15)		P(4)–Ag(2)–P(2)	122.36(2)
  P(1)–Ag(1)–P(3)	135.27(2)		N(6)#1–Ag(1)–Ag(2)	49.02(5)		N(4)–Ag(2)–Ag(1)	143.30(6)
  N(1)–Ag(1)–N(6)#1	89.98(8)		N(4)–Ag(2)–N(6)#1	90.12(7)		N(6)#1–Ag(2)–Ag(1)	53.18(5)
  P(1)–Ag(1)–N(6)#1	106.35(5)		N(4)–Ag(2)–P(4)	110.60(6)		P(4)–Ag(2)–Ag(1)	90.882(15)
  P(3)–Ag(1)–N(6)#1	104.00(5)		N(6)#1–Ag(2)–P(4)	119.96(5)		N(4)–Ag(2)–P(2)	102.54(6)
  N(1)–Ag(1)–Ag(2)	138.37(6)
  Symmetry code for 1: –x+1/2, y+1/2, –z+1/2

  3. RESULTS AND DISCUSSION

  3.1 Crystal structures

  The assembly of {Ag₂(dppm)₂[N(CN)₂]}₂[μ-N(CN)₂]₂ complex 1 is directed by the reaction of dppm, NaN(CN)₂ and Ag(CF₃SO₃) in a 1:1:1 ratio. The single-crystal X-ray crystallography reveals that 1 crystallizes in monoclinic system with P2₁/n space group. The ORTEP drawing of complex 1 is depicted in Fig. 1. The whole molecular structure can be considered as two binucleate moieties {Ag₂(dppm)₂[N(CN)₂]}⁺ linked by two bridging anionic ligands N(CN)₂⁻. For each binucleate moiety, the Ag₂ cores are doubly linked by dppm ligands with P donors. The coordination geometry around the Ag(Ⅰ) center can be described as a slightly distorted tetrahedron. Each Ag(Ⅰ) center is four-coordinated by two P atoms from dppm ligand and two N atoms from the anionic ligand N(CN)₂⁻. Four N(CN)₂⁻ ligands adopt two types of coordinating modes. Two of them exhibit a bidentate bridging mode connecting two {Ag₂(dppm)₂[N(CN)₂]}⁺ moieties, and the other two show a monodentate mode bound to Ag(Ⅰ) centers in different {Ag₂(dppm)₂[N(CN)₂]}⁺ moieties. As exhibited in Table 1, all the bond lengths and bond angles are comparable with those observed in other literatures with silver complexes^{[13, 14]}. The bond length ranges of Ag(Ⅰ) and coordinating atom are 2.4280(6)~2.4729(6) Å for Ag−P and 2.358(2)~2.538(2) Å for Ag−N, respectively. It is notable that the Ag···Ag distance is 3.0990(3) Å, which is much shorter than the sum of van der Waals radii (3.44 Å), indicating the presence of significant argentophilic interaction^[27]. This argentophilic interaction is found to be significantly influential on the photophysical properties of the silver complexes according to extensive research^[28]. Some N atoms in N(CN)^2- form moderate intensive hydrgen bond with C–H of adjacent phenyl rings, which falls in the range of 2.682(3)~2.746(8) Å. No aromatic stacking is observed in the crystal structure.

  Figure 1

  

  Figure 1.  (a) ORTEP drawing of the cation of complex 1 with atom labelling scheme showing 30% thermal ellipsoids (Hydrogen atoms are removed for clarity); (b) Demonstration of the skeleton of complex 1 (Hydrogen atoms and phenyl rings were removed for clarity)

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  3.2 IR spectrum analysis

  As shown in Fig. 2, the solid state IR spectrum of complex 1 was performed and the result is consistent with the single-crystal structure. The characteristic stretching vibration bands of dicyanamide were detected. Relative to v(N(CN)₂) in the free dicyanamide (2287, 2229, and 2181 cm⁻¹), five peaks at 2274, 2240, 2202, 2153 and 2139 cm⁻¹ can be attributed to the C‒N and C≡N stretching vibrations of dicyanamide ligand due to two different coordination modes as demonstrated in the crystal structure. Absorption peaks in the range of 500~1500 cm⁻¹ are attributed to the variations in the dppm ligand.

  Figure 2

  

  Figure 2.  IR spectrum of complex 1

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  3.3 Thermal analysis

  Thermogravimetric analysis (TGA) was carried out to study the thermal stability of complex 1 with desolvated sample for which the as-obtained crystal of complex 1 was heated at 60 ℃ for 3 hours under vacummizing condition. The resulting TGA curve is shown in Fig. 3. Complex 1 shows no obvious weight loss below relative high temperature of 250 ℃, indicating it is stable enough to hold its structure owing to the stable tetrahedral metal core with enough external supporting chelate ligands. From about 250 to 575 ℃, there are three steps of weight loss corresponding to the decomposition of two kinds of ligands with three coordination modes in 1. The final residue of 20.58% is close to the calculated value of 20.75% based on Ag₂O.

  Figure 3

  

  Figure 3.  Thermogravimetric analysis curve of the desovated complex 1

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  3.4 Luminescent property

  The temperature-independent luminescent property of complex 1 was characterized using a FLS920 fluorescence spectrometer from 298 to 77 K. As illustrated in Fig. 4, upon irradiation at 330 nm, the solid-state of fresh prepared complex 1 is non-emissive at room temperature, which is likely because the complicated molecular structure with suspended ligands increases the molecular flexibility so as to enhance the non-radiative transition. However, a bright blue emission centered around 450 nm gradually appears and enhances with cooling compound 1 to 77 K due to the increased molecular rigidity. As depicted in Table 2, the emission of complex 1 displays a slight red-shift of about 10 nm during cooling. Noteworthy, the lifetime of solid-state sample 1 is located at the microsecond range and significantly increased from 1.7 to 45.4 μs as the temperature decreases. When cooling, the non-radiative transition due to molecular rotation, vibration and collision is suppressed. Therefore, the luminescence is significantly increased. Recyclability of the thermochromic property has also been studied. As Fig. 5 shows, we measured three cooling cyscles of emission and lifetime changing of complex 1 and obtained similar results, indicating that this thermochromic property shows good recyclability.

  Figure 4

  

  Figure 4.  Emission spectra of cluster complex 1 in solid state from 298 to 77 K

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  Table 2

  

  Table 2.  Luminescent Data of Complex 1 in Solid State from 298 to 77 K

  DownLoad: CSV

  Temperature/K	298	250	200	150	100	77
  λem/nm	―	447	449	452	455	457
  τem/μs	―	1.7	5.6	18.1	33.9	45.4

  Figure 5

  

  Figure 5.  Recyclability of the thermochromic behavior of cluster complex 1

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  4. CONCLUSION

  In conclusion, a stable thermochromic silver(Ⅰ) complex {Ag₂(dppm)₂[N(CN)₂]}₂[μ-N(CN)₂]₂ (1) is synthesized and fully characterized. A mixed-ligand system including nitrogenous and phosphine ligand is utilized to assemble this complex. Single-crystal X-ray structure analysis reveals strong Ag···Ag interactions in the complex. Thermogra-vimetric analysis confirms the good thermal stability of complex 1. Interesting thermochromic property in the solid state of 1 is also observed. With the reduction of temperature from 298 to 77 K, a surprising emission enhancement as well as lifetime extension is detected. This work provides an approach to obtain thermochromic silver(Ⅰ) complexes via mixed-ligand system.

  
  
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• 

  Scheme 1  Synthetic route of complex {Ag₂(dppm)₂[N(CN)₂]}₂[μ-N(CN)₂]₂ (1)

  下载: 全尺寸图片 幻灯片
  

  Figure 1  (a) ORTEP drawing of the cation of complex 1 with atom labelling scheme showing 30% thermal ellipsoids (Hydrogen atoms are removed for clarity); (b) Demonstration of the skeleton of complex 1 (Hydrogen atoms and phenyl rings were removed for clarity)

  下载: 全尺寸图片 幻灯片
  

  Figure 2  IR spectrum of complex 1

  下载: 全尺寸图片 幻灯片
  

  Figure 3  Thermogravimetric analysis curve of the desovated complex 1

  下载: 全尺寸图片 幻灯片
  

  Figure 4  Emission spectra of cluster complex 1 in solid state from 298 to 77 K

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  Figure 5  Recyclability of the thermochromic behavior of cluster complex 1

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  Table 1.  Selected Atomic Distances (Å) and Bond Angles (°) of the Cluster Complex 1

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Ag(1)–N(1)	2.358(2)		Ag(1)–N(6)#1	2.538(2)		Ag(2)–N(6)#1	2.394(2)
  Ag(1)–P(1)	2.4280(6)		Ag(1)–Ag(2)	3.0990(3)		Ag(2)–P(4)	2.4387(6)
  Ag(1)–P(3)	2.4394(6)		Ag(2)–N(4)	2.389(2)		Ag(2)–P(2)	2.4729(6)
  Angle	(°)		Angle	(°)		Angle	(°)
  N(1)–Ag(1)–P(1)	112.25(6)		P(1)–Ag(1)–Ag(2)	89.642(15)		N(6)#1–Ag(2)–P(2)	105.27(6)
  N(1)–Ag(1)–P(3)	99.73(6)		P(3)–Ag(1)–g(2)	86.128(15)		P(4)–Ag(2)–P(2)	122.36(2)
  P(1)–Ag(1)–P(3)	135.27(2)		N(6)#1–Ag(1)–Ag(2)	49.02(5)		N(4)–Ag(2)–Ag(1)	143.30(6)
  N(1)–Ag(1)–N(6)#1	89.98(8)		N(4)–Ag(2)–N(6)#1	90.12(7)		N(6)#1–Ag(2)–Ag(1)	53.18(5)
  P(1)–Ag(1)–N(6)#1	106.35(5)		N(4)–Ag(2)–P(4)	110.60(6)		P(4)–Ag(2)–Ag(1)	90.882(15)
  P(3)–Ag(1)–N(6)#1	104.00(5)		N(6)#1–Ag(2)–P(4)	119.96(5)		N(4)–Ag(2)–P(2)	102.54(6)
  N(1)–Ag(1)–Ag(2)	138.37(6)
  Symmetry code for 1: –x+1/2, y+1/2, –z+1/2

  下载: 导出CSV

  Table 2.  Luminescent Data of Complex 1 in Solid State from 298 to 77 K

  Temperature/K	298	250	200	150	100	77
  λem/nm	―	447	449	452	455	457
  τem/μs	―	1.7	5.6	18.1	33.9	45.4

  下载: 导出CSV
•